The present invention relates to a flow-through electrolytic cell. More particularly, this invention relates to a flow-through cell utilizing porous anodic and cathodic electrodes for the preparation of aryl hydroquinones by the sequential oxidation and reduction of aryl compounds.
The synthesis of organic compounds by electrolysis is a relatively new technology. Electrolytic oxidation, reduction, and coupling of organic molecules have been carried out in the laboratory for more than a century, but only in the past fifteen to twenty years have organic chemicals been made on an industrial scale by electrochemical methods.
The proper combination of electrode and electrolyte is possibly the most important consideration for an organic electrochemical process. In electrochemical reductions, the cathode material may be critical in directing the electrochemistry toward the desired reaction products. Frequently, materials with high hydrogen-overvoltage such as lead, mercury, tin, zinc, cadmium, and graphite are required for satisfactory product selectivity and current efficiency. Because of its high hydrogen overpotential, lead is a potentially ideal cathode material for such processes. However, because lead has a low specific surface area and porosity the principal electrode reaction is usually hydrogen evolution. Increasing the specific surface area of a lead electrode would have an obvious advantage in this respect, but this advantage is usually limited by the rate of diffusion of the reacting species. It is therefore also important to have rapid movement of the electrolytic species through the electrode. To achieve mobility and efficient contact, small particles of lead could be uniformly distributed throughout a porous matrix. Some metals can be electrodeposited as small particles, but lead deposits as sheets or flakes, with preferential growth in the direction of the applied electric field. This anisotropic growth can be partially but not completely eliminated by electrodeposition from a solution containing a fluoroborate, silicofluoride, sulphamate, or pyrophosphate salt. Another possible technique would be the condensation of lead vapor in a porous matrix, but unfortunately the lead particles tend to form large aggregates on the surface of the matrix.
Known methods for the electrochemical oxidation or reduction of organic compounds include dissolving or suspending the organic compounds in an aqueous electrolyte solution, and passing the solution or suspension through an electrochemical cell. Such methods have the inherent disadvantage that the organic compounds may be at least partially oxidized to an oxidation level beyond that of the desired product, due primarily to the cell design. The product selectivity and current efficiency of such electrochemical methods may be lowered, and undesired by-products may be formed.